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The Biomechanics of
Muscle Actions
Instructor:
Alan Wiest
Presentation Objectives
• Define biomechanics
• Introduce lever systems
• Differentiate among the types of muscle
actions of the musculoskeletal system
• Determine the torque requirements
necessary to cause those muscle
actions
Terminology
• Biomechanics- the
application of mechanical
principles to biological
systems.
– “the physics of human
movement”
• Applications for a trainer:
– safety, technique,
intensity, client diversity,
research
Mechanical
and
Anatomical
Lever
Examples
Levers of the Musculoskeletal
System
• Muscles function by pulling
against bones
• Bones rotate about joints
– transmit force through the skin to
the environment (dumbbell, etc.)
• Body movements primarily act
through the bony levers of the
skeleton.
Lever System
(5 components of a lever system)
O
FM
MM
MR
>
(FM)(MM) = (FR)(MR)
<
•
•
•
•
•
O = fulcrum
FM = muscle force
FR = resistive force
MM = moment arm of the muscle force
MR = moment arm of the resistive force
FR
Muscle Actions
• Isometric Muscle Action:
– muscle stays the same length while
under tension
– contractile torque = resistive torque
(FM)(MM) = (FR)(MR)
Lever System
• Torque Equation:
(FR)(MR)
FM = ?
M
M
=1
0i
n.
(FM)(MM)
>
=
<
isometric
FM = ?
O
MR = 36 in.
FR = 40
lbs.
Torque Equation
• Solve for isometric action
>
(FM)(MM) = (FR)(MR)
<
(FM=?)(10in.) = (40lbs.)(36in.)
(10 in.)
(10 in.)
FM = 144 lbs.
Muscle Actions
• Concentric Muscle Action:
– muscle shortens while under tension
– contractile torque > resistive torque
– effort phase of dynamic exercises
– lifting resistance up against gravity
(FM)(MM) > (FR)(MR)
Lever System
• Torque Equation:
(FM)(MM)
>
=
<
concentric
FM > 144 lbs.
(FR)(MR)
O
FM
MM
MR
FR
Muscle Actions
• Eccentric Muscle Action:
– muscle lengthens while under
tension
– contractile torque < resistive torque
– negative phase of dynamic exercises
– controlled lowering of resistance
against gravity
(FM)(MM) < (FR)(MR)
Lever System
• Torque Equation:
(FM)(MM)
>
=
<
eccentric
(FR)(MR)
FM < 144 lbs.
O
FM
MM
MR
FR
Isometric Muscle Action
-no movement
-contractile torque =
resistive torque
Concentric Muscle Action
-muscle shortens
-contractile torque >
resistive torque
Eccentric Muscle Action
-muscle lengthens
-contractile torque <
resistive torque
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